The Astrophysical Journal Supplement Series, 141:311–335, 2002 August # 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A.
VSOP SPACE VLBI AND GEODETIC VLBI INVESTIGATIONS OF SOUTHERN HEMISPHERE RADIO SOURCES S. J. Tingay1 CSIRO Australia Telescope National Facility, Paul Wild Observatory, Narrabri, NSW 2390, Australia; [email protected] J. E. Reynolds, A. K. Tzioumis, David L. Jauncey, and J. E. J. Lovell CSIRO Australia Telescope National Facility, P.O. Box 76, Epping, NSW 2121, Australia R. Dodson, M. E. Costa, and P. M. McCulloch Department of Physics, University of Tasmania, G.P.O. Box 252-21, Hobart, Tasmania 7001, Australia P. G. Edwards and H. Hirabayashi Institute of Space and Astronautical Science, Sagamihara, Kanagawa 229, Japan D. W. Murphy and R. A. Preston Jet Propulsion Laboratory, California Institute of Technology, MS 238-332, 4800 Oak Grove Drive, Pasadena, CA 91109 B. G. Piner Whittier College, Department of Physics, 13406 Philadelphia Street, Whittier, CA 90608 G. D. Nicolson and J. F. H. Quick Hartebeesthoek Radio Astronomy Observatory, P.O. Box 443, Krugersdorp 1740, Transval, South Africa and H. Kobayashi and K. M. Shibata National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan Received 2002 January 7; accepted 2002 March 19
ABSTRACT We present images from VLBI Space Observatory Programme (VSOP) observations of 14 compact extra- galactic southern hemisphere radio sources, including a description of the observations, the data reduction techniques, and the parameters of the resulting images and model fits. These images provide the highest reso- lution information to date for many of these objects. Comparisons are made between VSOP and previous ground-based VLBI results, including images from data extracted from the geodetic VLBI archive at the United States Naval Observatory. From the VSOP data, we find that the two radio galaxies observed have lower peak brightness temperatures than the 12 quasars. Also, these data show (1) no evidence for obvious differences between the brightness temperature distributions of gamma-ray–loud and gamma-ray–quiet radio-loud active galactic nuclei and (2) no evidence for obvious correlations between brightness temperature and spectral index, radio polarization, flux density, or month timescale modulation index. These results are consistent with previous work by Lister, Tingay, & Preston, who found that the only observable significantly correlated with VSOP-derived brightness temperature is intraday variability, which is strongly correlated with many relativistic beaming indicators. For one source, PKS 1127 145, we undertake a detailed investiga- tion of the milliarcsecond-scale component positions as a function of time, taking data from the literature and the current work, to estimate proper motions. As a result, we suggest that two components previously reported as stationary, C1 and C2, have apparent transverse speeds of (9:1 3:8) and (5:3 2:3) h 1c, respec- tively. We also make the first investigation of the apparent motion in the nearest GHz-peaked spectrum radio galaxy, PKS 1718 649, finding an upper limit on the apparent separation speed of 0.08c. Comparison of geodetic VLBI and VSOP data show no significant detection of component motion in PKS 0208 512, (2:4 3:1) h 1c, and only a tentative detection in PKS 0537 441, (2:8 2:2) h 1c. A significant detection of component motion is found in PKS 1610 771, solely from the geodetic VLBI data, (9:4 3:5) h 1c. Subject headings: galaxies: active — galaxies: jets — quasars: general — radiation mechanisms: nonthermal — radio continuum: galaxies — techniques: interferometric
1. INTRODUCTION many organizations and individuals. ISAS successfully The VLBI Space Observatory Programme (VSOP) is led launched the first dedicated space-based Very Long Baseline Interferometry (VLBI) radio telescope, named HALCA by the Japanese Institute of Space and Astronautical (Highly Advanced Laboratory for Communications and Science (ISAS) and relies upon the coordinated effort of Astronomy), into Earth orbit on 1997 February 12. Since that time HALCA has observed many compact extragalac- tic radio sources, mainly active galactic nuclei (AGNs), sup- 1 Also Jet Propulsion Laboratory, California Institute of Technology, ported by ground arrays such as the Very Long Baseline MS 238-332, 4800 Oak Grove Drive, Pasadena, CA 91109. Array (VLBA), the European VLBI Network (EVN), and 311 312 TINGAY ET AL. Vol. 141 the Southern Hemisphere Long Baseline Array (LBA). The observations together in this paper, since in all cases the HALCA orbit is such that the longest baseline formed in observational and data reduction techniques are very simi- combination with these ground telescopes is approximately lar. Also presented are new results for some of these sources 2.6 times the diameter of the Earth, providing a correspond- based on geodetic VLBI data. We derive and discuss basic ing resolution increase beyond the maximum possible from parameters of the sources and the samples. Subsequent the ground alone, in the VSOP bands at 6 and 20 cm. A papers will deal with the detailed statistical interpretation of major justification for space VLBI is that the maximum the data for the various classes of source, drawing from the brightness temperature an interferometer can measure accu- current VLBI and radio data and the wider literature. rately is proportional to the physical (projected) baseline The VSOP images presented here represent the highest length squared. Thus, VSOP has the potential to measure angular resolution images ever obtained for the majority of brightness temperatures up to 6–7 times greater than an these radio sources. Throughout this paper q0 ¼ 0:1 has Earth-based interferometer. Full descriptions of the VSOP been used, unless stated otherwise. mission can be found in Hirabayashi et al. (1998, 2000a). We have used the VSOP mission facilities to observe com- pact extragalactic radio sources at southern declinations, as 2. OBSERVATIONS, DATA REDUCTION, the logical extension to ground-based investigations that AND RESULTS have taken place over the last 10 yr (Tingay et al. 1996a, 2.1. VSOP Data 1996b, 1997, 1996a, 1998b, 1998c, 2000; Tornikoski et al. 1999). Sources of various AGN classes have been observed Table 1 lists the sources observed by the VSOP mission, as part of this work, corresponding to several separate pro- the appropriate observing proposal, as listed above, and posals to the VSOP mission submitted during the first two basic data such as average 4.8 GHz flux density, average announcement of opportunity (AO) periods (1997–1999). spectral index between 4.8 and 8.6 GHz, average percentage The proposals include the following: gamma-ray–loud and polarization at 4.8 GHz, redshift, and object identification. gamma-ray–quiet AGNs with VSOP and SHEVE at 5 GHz The flux density, spectral index, and polarization data are (V115, AO1), VSOP and the bursting gamma-ray source averages obtained from Australia Telescope Compact PKS 2255 282 (W025, AO2), continuous monitoring of Array (ATCA) monitoring observations between 1996 July ecliptic pole source PKS 0637 752 (W029, AO2), gamma- and 2000 February, using the 6 km maximum baseline of ray–loud and gamma-ray–quiet AGNs with VSOP at 5 the ATCA (S. J. Tingay et al. 2002, in preparation). Also GHz (W030, AO2), and high linear resolution imaging of given in Table 1 are the rms deviations from the mean flux z < 0:06 sources (W062, AO2). density at 4.8 GHz observed between 1996 and 2000 and the Gamma-ray–loud sources for V115 were selected to be ratio of rms deviation to mean flux density (modulation those south of declination ¼ 40 in the first Energetic index), derived from the ATCA data. ATCA data for PKS Gamma Ray Experiment Telescope (EGRET) catalog 1127 145 and PKS 2152 699 are not available. Data for (Fichtel et al. 1994). The V115 gamma-ray quiet sources these two sources in Table 1 are from Stickel et al. (1994). were selected to be south of ¼ 40 , EGRET nondetec- A total of 15 successful VSOP observations of 14 sources tions according to Fichtel et al. (1994) and have radio (flux have been made from a list of 24 sources proposed. A small density and spectral index) and optical properties (polariza- number of scheduled observations were unsuccessful and tion) that are typical of the gamma-ray–loud radio source may be rescheduled in the future. However, at the time of population. The additional sources selected for W030 were writing it appears unlikely that observations of a significant selected on the same basis, except that the declination limit number of the remaining 10 sources will be obtained before for the gamma-ray–loud sources was raised to ¼ 10 ,to the end of the VSOP mission, owing to tightened observing include more sources. Also, over the years since the analysis constraints for the spacecraft. Table 2 gives details of the of Fichtel et al. (1994), the EGRET source list has been observations for each source, including the UT date of modified, with the result that some of the originally selected observation, the array of ground radio telescopes used, the sources have switched categories. For the work in this paper tracking station(s) used to track HALCA, the correlator we use the identifications given in the most recent EGRET where the data were initially processed, the frequency of catalog, that of Hartman et al. (1999). The gamma-ray observation, and the approximate time spent on-source for source PKS 2255 282 was selected individually as an inter- the ground antennas and the space antenna. The on-source esting single source because or the gamma-ray and milli- time refers to the successfully correlated and fringe-fitted meter flares that brought it to prominence (Tornikoski et al. time ranges. Some amount of data is lost during imaging— 1999). the data that do not fulfil self-calibration requirements— The low-redshift sources were selected from the list of and thus the time range of data finally represented in the 1 Jy sources at 5 GHz (and jbj > 10 ) of Stickel, Meisen- images and (u; v) coverages may be different from those heimer, & Ku¨hr (1994); of 527 sources, 71 lie at redshifts less given in Table 2. All observations used a 32 MHz band- than z ¼ 0:06, yielding 1 mas 1 pc. From the 71 sources width, split into two 16 MHz channels; the frequency of we took 29 as candidate sources for our sample (those with observation is given as the center of the 32 MHz band. flat radio spectra and those with total flux densities greater Data processed at the Dominion Radio Astrophysical than 4 Jy). We searched the literature for these sources and Observatory (DRAO) correlator in Penticton, Canada, found that 12 of these contain in excess of 1 Jy on moder- were recorded on S2 format tapes, at both the ground tele- ate length VLBI baselines (for example, baselines within scopes and tracking stations. Data processed at the Australia). For observation from the southern hemisphere, National Radio Astronomy Observatory (NRAO) VLBA we selected six of these sources, those south of ¼ 0 . correlator in Socorro, New Mexico, were recorded on A total of 14 sources have been observed as part of these VLBA format tapes. Occasionally non-NRAO telescopes projects. We present the VSOP results for all of these not equipped with a VLBA format recording terminal No. 2, 2002 VSOP RADIO SOURCE OBSERVATIONS 313
TABLE 1 Sources Observed by the VSOP Mission
8:6 Source VSOP Experiment S4:8 GHz 4:8 mp rms rms/S4:8 GHz z ID PKS 0208 512...... V115 3.0 0.12 1.4 0.23 0.08 1.003 QG PKS 0438 436...... V115 3.2 0.10 0.6 0.32 0.11 2.852 Q PKS 0537 441...... V115 3.4 0.12 1.3 1.0 0.29 0.894 QG PKS 0637 752...... V115/W029 6.3 0.11 1.2 0.12 0.02 0.651 Q PKS 1104 445...... W030 2.6 0.05 2.4 0.23 0.10 1.598 Q PKS 1127 145...... W030 5.5 1.03 ...... 1.187 Q PKS 1424 418...... W030 5.0 0.24 2.7 0.17 0.04 1.522 QG PKS 1610 771...... W030 3.0 0.24 3.6 0.16 0.06 1.710 Q PKS 1622 253...... W030 1.8 0.17 1.2 0.47 0.27 0.786 QG PKS 1622 297...... W030 2.4 0.17 4.6 0.32 0.13 0.815 QG PKS 1718 649...... W062 4.6 0.38 <0.2 0.07 0.02 0.014 GPS PKS 2152 699...... W062 12.3 1.44 ...... 0.028 FRII PKS 2255 282...... W025 3.4 0.90 0.33 0.9 0.26 0.926 QG PKS 2355 534...... V115 1.5 0.09 3.1 0.03 0.05 1.006 Q
Note.—Source: Parkes catalog designation; S4:8 GHz: average flux density at 4.8 GHz on the 6 km ATCA baseline in Jy, 8:6 between 1996 July and 2000 February; 4:8: average spectral index between 4.8 and 8.6 GHz (S / ) from ATCA 6 km data; mp: average percentage linear polarization at 4.8 GHz from ATCA 6 km data; rms: rms deviation of 4.8 GHz flux den- sity around S4:8 GHz, in Jy, from ATCA 6 km data; rms/S4:8 GHz: ratio of rms to S4:8 GHz; z: redshift; and ID: gamma-ray– quiet quasar (Q), gamma-ray–loud quasar (QG), Fanaroff-Riley type II radio galaxy (FRII), and GHz-peaked spectrum (GPS) galaxy. recorded data on S2 format tapes and shipped them to the order using the task MSORT and then indexed using S2 to VLBA format translation machine at the National INDXR. For data from the VLBA and Mitaka correlators, Astronomical Observatory of Japan (NAOJ) in Mitaka, the task ACCOR was used to correct the correlated ampli- Japan. The translated tapes were then correlated with tudes for errors in the sampler thresholds. VLBA format tapes recorded at other telescopes. Data Calibration information for the telescopes, in the form of processed at the NAOJ correlator in Mitaka were recorded system temperatures and antenna gains, were collected from on S2 format tapes and translated at Mitaka into the VSOP the participating observatories. For data from all correla- format used at the Mitaka correlator. tors, the calibration information was loaded into AIPS After correlation, the data were imported into the Astro- using the task ANTAB and amplitude calibration tables nomical Image Processing Software (AIPS) for further were produced using APCAL. For the data from later processing. The data were initially sorted into time-baseline observations using the VLBA, calibration information for
TABLE 2 Details of the Observations for Each Source
Time (hr) Source UT Date Ground Telescopes Tracking Stations Correlator (GHz) Ground Space
PKS 0208 512...... 1998 Jun 10 Mp Hh At Ho, Mk Nz Penticton 4.946 14 7 PKS 0438 436...... 1998 Mar 7 Mp Hh At Mk Tz Rz Mitaka 4.949 10 5 PKS 0537 441...... 1998 Mar 1 Mp Hh Ho Fd Ov Mk Gz Mitaka 4.946 14 3 PKS 0637 752...... 1997 Nov 21 Mp Hh At Gz Tz Penticton 4.976 12 6 PKS 0637 752...... 1999 Aug 19 Mp Hh At Ho Rz Uz Penticton 4.816 9 8 PKS 1104 445...... 1999 May 27 Mp Hh At Ho So Rz Uz Penticton 4.946 7 4 PKS 1127 145...... 1999 Apr 24 Mp VLBA-Mk Tz Nz Socorro 4.816 8 4 PKS 1424 418...... 1999 Aug 1 Mp Fd Ov Mk Kp La Pt Sc Gz Socorro 4.816 5 3 PKS 1610 771...... 1999 Apr 5 Mp Hh At Ho Gz Rz Tz Penticton 4.946 10 8 PKS 1622 253...... 2000 May 22 VLBA Tz Nz Socorro 4.816 4 2 PKS 1622 297...... 2000 Apr 25 VLBA Tz Nz Socorro 4.816 4 2 PKS 1718 649...... 1999 Mar 25 Hh At Ho Tz Nz Penticton 4.946 6 0.25 PKS 2152 699...... 1999 Apr 8 Hh At Ho Tz Nz Uz Penticton 4.946 8 2 PKS 2255 282...... 1999 May 18 Mp At Ho VLBA Rz Socorro 4.976 2 2 PKS 2355 534...... 1998 Jun 6 Mp Hh Gz Rz Penticton 4.946 6 3
Note.—Ground telescopes: Mp ¼ Mopra (Australia Telescope National Facility; ATNF); Hh ¼ Hartebeesthoek (Hartebeesthoek Radio Astronomy Observatory; HRAO); At ¼ Australia Telescope Compact Array (ATNF); Ho ¼ Hobart (University of Tasmania); Sh ¼ Sheshan (Shanghai Astronomical Observatory; SAO); Fd ¼ Fort Davis (National Radio Astronomy Observatory; NRAO); Ov ¼ Owens Valley (NRAO); Mk ¼ Mauna Kea (NRAO); Kp ¼ Kitt Peak (NRAO); La ¼ Los Alamos (NRAO); Pt ¼ Pie Town (NRAO); Sc ¼ Saint Croix (NRAO); VLBA ¼ full VLBA (NRAO) Tracking Stations: Nz ¼ Green Bank (NRAO); Rz ¼ Robledo (National Aeronautics and Space Administration; NASA); Gz ¼ Goldstone (NASA); Tz ¼ Tidbinbilla (NASA); Uz ¼ Usuda (ISAS) CORR (Correlator): Penticton ¼ S2 (Dominion Radio Astro- physical Observatory; DRAO); Mitaka ¼ VSOP (National Astronomical Observatory of Japan; NAOJ); and Socorro ¼ VLBA (NRAO). 314 TINGAY ET AL. Vol. 141 the VLBA antennas was attached to the ðu; vÞ data at the had a relatively large number of antennas, seven or more, correlator. For the ground antennas, long experience shows and the data had a high signal-to-noise ratio, then ampli- that this a priori amplitude calibration data can usually be tude self-calibration was performed on shorter timescales. considered to be uncertain by less than 20%, and often by For imaging, 512 512 pixel images were used, with 0.05 less than 10%. This expectation is borne out during imaging. mas pixel sizes. When amplitude self-calibration is possible, the corrections Figures 1–8 show the final self-calibrated images for the to the amplitudes on the ground antennas are very seldom observations listed in Table 2, along with the corresponding greater than 20%, more often around the 5%–15% value. ðu; vÞ coverages. Given the oftentimes sparse and irregular The HALCA nominal 5 GHz system temperature of 90 K is ðu; vÞ coverages resulting from space VLBI observations, stable and with the nominal 5 GHz gain of 0.0062 (Moellen- errors in the images due to nonuniqueness in the deconvolu- brock et al. 2000) gives a calibration that appears to be at tion step can be significant, as is the case for data from any least as good as the ground antennas; amplitude self- sparse array. This means that some structures seen in VSOP calibration (where possible) yielded corrections seldom images cannot be fully trusted; generally low surface bright- greater than 10% for HALCA. ness extended emission suffers worst from this effect. The Once the amplitude calibration was applied, all data were effects of deconvolution errors in VSOP data are discussed fringe-fitted using the task FRING. The two 16 MHz chan- in detail by Lister et al. (2001a). Since the fainter extended nels were fringe-fitted independently since no calibration structures in our VSOP images are sometimes difficult to was available to remove the instrumental phase and delay interpret because of these effects, we do not analyze them in offsets between the two channels for data from HALCA. detail in this paper. We primarily limit ourselves here to an However, all sources except PKS 1718 649 were easily analysis of the highest brightness temperature features in strong enough to be detected in each single 16 MHz channel, our images. The study of high brightness temperature emis- on all projected baselines. Thus, in general, no sensitivity or sion is one of the main justifications for space VLBI mis- resolution penalty was incurred because of this limitation. sions, providing information that cannot, in general, be For PKS 1718 649, the source was detected only during obtained from ground-based VLBI observations. fringe-fitting on the shortest space baselines or those base- Figure 9 shows plots of the flux density as a function of lines at a favorable position angle in the (u; v) plane, indicat- distance in the (u; v) plane, for each source. These plots con- ing that the components are heavily resolved on longer tain comparisons of the final self-calibrated data to the visi- baselines, at the sensitivity provided by a single 16 MHz bilities generated by the final models, showing the fit of the channel. For this source many different fringe-fitting time- clean components to the data. scales were attempted, and the degree of continuity of the After imaging, the ðu; vÞ plane data for each source were delay and rate solutions were used to confirm that the model-fitted using the technique outlined by Tingay et al. source was not detected. (2001). For the brightest, most compact component of each If ancillary information was available from the correlator source (which in most cases can be identified as the core), noting the magnitude and sign of the fringe delay for partic- the group of clean components representing that compo- ular antennas, those delays were corrected using the task nent were replaced with a single elliptical Gaussian compo- CLCOR before fringe-fitting, so that smaller delay windows nent with six free parameters. The Gaussian model was could be used in fringe-fitting. Solution intervals in fringe- fitted to the amplitude and phase data after up-weighting fitting varied between 2 and 10 minutes and were chosen to the space visibilities relative to the ground visibilities, in maximize the signal-to-noise ratio of the fitted fringes. A analogy to uniform weighting schemes in imaging (Tingay signal-to-noise ratio cutoff of 5–7 was used to ensure the et al. 2001). The resulting core component model parame- reliability of fringes found by FRING. ters for each source are given in Table 3. The results of fringe-fitting—the fitted delays, rates, and Errors on the parameters of the Gaussian model compo- phases—were applied to the data and the visibilities aver- nents were estimated, using the DIFWRAP software aged across each 16 MHz channel using the task SPLIT. (Lovell 2000c), as described in Tingay et al. (2001). In The two-channel, frequency-averaged data were written to particular, the parameters analyzed were the core flux disk as FITS files and then imported into DIFMAP density and size, the parameters required to estimate the (Shepherd, Pearson, & Taylor 1994) for imaging and ðu; vÞ core brightness temperatures, according to the following plane model fitting. expression, In DIFMAP, the data were vector-averaged into 30 s points and phase self-calibrated to a 1 Jy point-source 1:22S T ¼ ; model that was then discarded. The amplitude and phase b;G ab 2 data were edited then Fourier-transformed into the image plane using a uniform weighting scheme, cleaned (initially where Tb;G, S, a, and b are described in Table 3. The fre- using a tight window around the brightest emission, then quency of observation is (in GHz). The brightness temper- slowly extending the windows to include fainter emission), atures, estimated from the parameters of the Gaussian and self-calibrated in phase until the accumulated clean models, in the observer’s and source comoving frames (cor- component model matched the visibility amplitudes on the responding to the source redshift), and the brightness tem- shorter baselines and no obvious residual flux remained to perature errors, estimated using DIFWRAP (these error be cleaned. This sometimes entailed switching between uni- estimates also take account of the approximate 10% error in form weighting and natural weighting, to detect the the flux density scale), are also given in Table 3. For compo- extended emission of some sources, in order to fit the short- nents with estimated lower limits on the component sizes est baselines. Finally, if more than four antennas were (upper limits on the brightness temperature) from the above present in the data set, amplitude self-calibration over the analysis, these estimates are consistent with theoretical esti- timescale of the observations was performed. If the array mates of the smallest possible measurable sizes, given a par- No. 2, 2002 VSOP RADIO SOURCE OBSERVATIONS 315
PKS 0208-512
PKS 0438-436
Fig. 1.—Top: PKS 0208 512 observed on 1998 June 10 at 4.946 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 0.55 Jy beam 1; contours are 4%, 4%, 8%, 16%, 32%, and 64% of peak; and beam size is 0:55 0:22 mas at P.A. 40=2. Bottom: PKS 0438 436 observed on 1998 March 7 at 4.946 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 1.02 Jy beam 1; contours are 2%, 2%, 4%, 8%, 16%, 32%, and 64% of peak; and beam size is 0:53 0:21 mas at P.A. 25=1.
ticular integrated signal-to-noise ratio, maximum baseline VSOP data constrain the upper limit to the size of the core length, and integrated flux density, as calculated by Lovell component very well. Sensitivity to the brightness tempera- et al. (2000a). ture lower limit and insensitivity to the upper limit for core 7 Table 3 shows that half (14) of the brightness temperature components is commonplace when dealing with VSOP data, estimates for the core components admit only a lower limit as evidenced by the similar results obtained from a VSOP 7 (12 of the quasars). This is because during the error analysis study of the Pearson-Readhead VLBI sample (Tingay et al. the visibility data were often found to be consistent with a 2001; Lister et al. 2001a; Lister, Tingay, & Preston 2001b) core component of zero extent in at least one dimension, and the VSOP Survey of compact extragalactic radio either a point-source component or a one-dimensional sources (Hirabayashi et al. 2000b). Gaussian. In either case, the solid angle subtended by the Figure 10 shows the brightness temperature estimates in core component is zero, formally giving an infinite bright- the observer’s frame and in the source frame, with errors, ness temperature. In all cases lower limits to the core for the sources listed in Table 3. Gamma-ray sources are brightness temperatures were found, indicating that the those identified by the EGRET instrument at energies above 316 TINGAY ET AL. Vol. 141
Fig. 2.—Top: PKS 0537 441 observed on 1998 March 1 at 4.946 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 0.50 Jy beam 1; contours are 2%, 2%, 4%, 8%, 16%, 32%, and 64% of peak; and beam size is 0:63 0:20 mas at P.A. 47=3. Bottom: PKS 0637 752 observed on 1997 November 21 at 4.976 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 0.73 Jy beam 1; contours are 5%, 5%, 10%, 20%, 40%, and 80% of peak; and beam size is 0:50 0:19 mas at P.A. 87=2.
100 MeV, as listed in the third EGRET catalog (Hartman et nomical information on source structure, although the data al. 1999). A comparison of the brightness temperature dis- quality for astronomical applications is below that pro- tributions of the gamma-ray–loud and gamma-ray–quiet duced by VLBI arrays dedicated to astronomical measure- sources is discussed in x 4. ments, and the dynamic ranges and sensitivities of the corresponding images are lower. Examples of astronomical 2.2. Geodetic VLBI Data results from the USNO geodetic database appear in We have extracted geodetic VLBI data from the United Tateyama et al. (1999) and Piner & Kingham (1998). States Naval Observatory (USNO) database for three of the Table 4 lists the geodetic VLBI observations extracted sources from Table 1, PKS 0208 512, PKS 0537 441, and from the archive at the USNO correlator in Washington PKS 1610 771. Data for a fourth source, PKS 0637 752, DC. For each observation, data have been obtained simul- have been described elsewhere (Schwartz et al. 2000; Lovell taneously at 2.3 and 8.4 GHz. Since each individual geodetic et al. 2000b). The USNO database is used primarily for geo- experiment contains only a relatively small amount of data, detic work, but the VLBI data also contain useful astro- we have combined data from experiments over periods of No. 2, 2002 VSOP RADIO SOURCE OBSERVATIONS 317
PKS 0637-752
PKS 1104-445
Fig. 3.—Top: PKS 0637 752 observed on 1999 August 19 at 4.816 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 0.87 Jy beam 1; contours are 2%, 2%, 4%, 8%, 16%, 32%, and 64% of peak; and beam size is 0:41 0:25 mas at P.A. 66=5. Bottom: PKS 1104 445 observed on 1999 May 27 at 4.946 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 0.41 Jy beam 1; contours are 4%, 4%, 8%, 16%, 32%, and 64% of peak; and beam size is 0:49 0:23 mas at P.A. 16=5. time ranging up to 20 days, to produce imagable data sets. the self-calibrated data subsequent to the imaging process, Table 4 indicates the time ranges over which different data with further self-calibration to ensure the best fit to the data. sets have been combined to produce single imagable data A typical ðu; vÞ coverage in shown in Figure 11. sets. These data have been fringe-fitted and calibrated as At 2.3 GHz the beam sizes for the observations listed in described in Piner & Kingham (1998) using AIPS, with Table 4 are typically between 1.5 and 2.5 mas and the rms imaging and model fitting undertaken in DIFMAP. For noise level in the resulting images ranges between 10 and 50 imaging, 128 128 pixel maps have been used with 0.5 mas mJy beam 1. At 8.4 GHz, the beam sizes are typically pixels for the 2.3 GHz data and 0.1 mas pixels for the 8.4 between 0.4 and 0.6 mas, and the rms noise level in the GHz data. Uniform weighting has been used for all images images also ranges between 10 and 50 mJy beam 1. with phase self-calibration but no amplitude self-calibra- The geodetic VLBI images can be seen in Figures 12, 13, tion. Circular Gaussian model components have generally and 14. Figure 12 shows the series of five images of PKS been used in model fitting. The models have been fitted to 0208 512 at 2.3 and 8.4 GHz. Figure 13 shows the series of 318 TINGAY ET AL. Vol. 141
Fig. 4.—Top: PKS 1127 145 observed on 1999 April 24 at 4.816 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 0.42 Jy beam 1; contours are 2%, 2%, 4%, 8%, 16%, 32%, and 64% of peak; and beam size is 0:92 0:23 mas at P.A. 10=2. Bottom: PKS 1424 418 observed on 1999 August 01 at 4.816 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 0.88 Jy beam 1; contours are 1%, 1%, 2%, 4%, 8%, 16%, 32%, and 64% of peak; and the beam size is 1:29 0:29 mas at P.A. 22=4. four images of PKS 0537 441 at 2.3 and 8.4 GHz. Figure beamwidth has been adopted, in recognition of the likely 14 shows the series of seven images of PKS 1610 771 at 2.3 errors due to the ðu; vÞ coverage. The separation between and 8.4 GHz. the core and jet components from the 1992 November The PKS 0208 512 2.3 GHz geodetic data can be model- observation is consistent with the new results at 2.3 GHz fitted at each epoch with a dual-component model, repre- from the geodetic archive data. There is no evidence for an senting a core and an extended jet component along a increase in the separation between the core and jet compo- position angle of approximately 130 (Table 5). Figure 15 nents between 1992 November and 1998 June. A weighted shows the separation between the core and jet components linear least-squares fit to the data give the apparent separa- as a function of time. For comparison, also shown are the tion speed to be 0:07 0:09 mas yr 1, corresponding to 1 component separations from the 4.8 GHz observation of app ¼ð2:4 3:1Þ h , where app is the apparent transverse Tingay et al. (1996a) of 1992 November and the 4.9 GHz speed of separation, in units of the speed of light and the VSOP space VLBI observation of 1998 June (Fig. 1). The Hubble constant is H0 ¼ 100 h. component separation from the VSOP data has been mea- At 8.4 GHz, the PKS 0208 512 data at only two of the sured from the image in DIFMAP, and an error of one five epochs required a two-component model, representing No. 2, 2002 VSOP RADIO SOURCE OBSERVATIONS 319
Fig. 5.—Top: PKS 1610 771 observed on 1999 April 05 at 4.946 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 0.15 Jy beam 1; contours are 10%, 10%, 20%, 40%, and 80% of peak; and beam size is 0:44 0:23 mas at P.A. 63=3; Bottom: PKS 1622 253 observed on 2000 May 22 at 4.816 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 0.37 Jy beam 1; contours are 2%, 2%, 4%, 8%, 16%, 32%, and 64% of peak; and beam size is 0:93 0:35 mas at P.A. 8=7. core and jet (Table 5). At three epochs only a single- an extension to the core toward the north, offset 90 from component model was required to fit the data, representing the extension to the core we see in the current data. The 4.9 only the core (Table 5). We suggest that this is because at GHz observation of PKS 0537 441 made with the VSOP the higher frequency the extended jet component may be data (Fig. 2) clarifies the situation; it shows that the jet ini- more difficult to detect with the limited data, owing to reso- tially emerges from the core at a position angle close to 90 lution and spectral index effects. At the two epochs where and undergoes an apparent bend toward the north. The two-component models were used, the separation and posi- extension close to the core is apparent only in the highest tion angle between the components were consistent, within resolution images, the VSOP and geodetic images. The the uncertainties, with the 2.3 GHz model fits. extension to the north is weak and detected only in the high- PKS 0537 441 at 8.4 GHz is resolved into a core and jet sensitivity images, the images of Tingay et al. (1996a), and component with approximately 0.4 mas separation at a the VSOP image. position angle of approximately 90 and has been model- The 8.4 GHz images of PKS 0537 441 give valuable fitted as such (Table 6). The only VLBI images of PKS confirmation that the high-resolution structure shown in 0537 441 previously published (Tingay et al. 1996a) show the VSOP image is correct. Figure 15 shows the separa- 320 TINGAY ET AL. Vol. 141
PKS 1622-297
PKS 1718-649
Fig. 6.—Top: PKS 1622 297 observed on 2000 April 25 at 4.816 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 0.93 Jy beam 1; contours are 1%, 1%, 2%, 4%, 8%, 16%, 32%, 64% of peak; and beam size is 0.96 0:35 mas at P.A. 20 . Bottom: PKS 1718 649 observed on 1999 March 25 at 4.946 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 0.62 Jy beam 1; contours are 2%, 2%, 4%, 8%, 16%, 32%, and 64% of peak; and beam size is 1:69 0:51 mas at P.A. 20=3. tion of the jet component from the core as a function of entirely due to the VSOP data, at a different frequency time. Also shown is the separation as measured from the and resolution from the geodetic data. VSOP image at 4.9 GHz, from 1998 March. As for PKS At 2.3 GHz the resolution of the PKS 0537 441 images is 0208 512, the separation from the VSOP data was mea- not high enough to resolve the core and jet components, and sured from the image in DIFMAP and one beamwidth is the sensitivity of the observations is not high enough to adopted as the error. No evidence for a change in separa- detect the northern component. Each 2.3 GHz data set can tion over the course of the geodetic observations is be adequately model-fitted with a single circular Gaussian apparent. However, comparing the geodetic data to the component (Table 6). VSOP data may allow a better estimate of the component PKS 1610 771 is resolved at both 2.3 and 8.4 GHz. At proper motion. A weighted linear least-squares fit to the 2.3 GHz, this source consists of a bright unresolved source separation as a function of time gives an apparent speed and an extension along a position angle of approximately of 0:09 0:07 mas yr 1, corresponding to 40 ; 2.3 GHz model fits are given in Table 7 (we have 1 app ¼ð2:8 2:2Þ h , only a very tentative detection of relaxed the requirement that only circular Gaussian compo- motion, especially given that the separation change is nents be used in the 2.3 GHz model fits for PKS 1610 771, No. 2, 2002 VSOP RADIO SOURCE OBSERVATIONS 321
PKS 2152-699
PKS 2255-282
Fig. 7.—Top:: PKS 2152 699 observed on 1999 April 08 at 4.946 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 0.13 Jy beam 1; contours are 2%, 2%, 4%, 8%, 16%, 32%, and 64% of peak; and beam size is 0:98 0:23 mas at P.A. 32=9. Bottom: PKS 2255 282 observed on 1999 May 18 at 4.976 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 2.13 Jy beam 1; contours are 1%, 1%, 2%, 4%, 8%, 16%, 32%, and 64% of peak; and beam size is 0:77 0:25 mas at P.A. 13=8.
in order to efficiently model the extended structure). No two components as a function of time is shown in Figure 15. consistent evolution of the source structure can be seen at A weighted linear least-squares fit to these separations gives 2.3 GHz. a proper motion of 0:19 0:07 mas yr 1, corresponding to 1 At 8.4 GHz, PKS 1610 771 is resolved into two compact an apparent transverse speed of app ¼ð9:4 3:5Þ h . components that can clearly be seen to separate, on inspec- These dual-frequency geodetic images of PKS 1610 771 tion of Figure 14, also confirming the structural position could be interpreted as follows. The weak component at 8.4 angle seen at 2.3 GHz. It should be noted that at 1993.44, GHz is actually the stationary core of the source, and the the flux densities of these two compact components appear strong component (fixed at the phase center in the 8.4 GHz to be reversed, probably owing to a phase ambiguity due images) is the component in motion, toward the northwest. to a low signal-to-noise ratio and sparse ðu; vÞ coverage. The Both components in the 8.4 GHz images contribute to the model fits of the 8.4 GHz data show that at each epoch extended component at the phase center of the 2.3 GHz (except for the first, 1993.09), the best fit is achieved by a images, with further extended jet emission detected to the two-component model (Table 7). The separation of these northwest. 322 TINGAY ET AL.
PKS 2355-534
Fig. 8.—PKS 2355 534 observed on 1998 June 06 at 4.946 GHz, image (left) and ðu; vÞ coverage (right). The peak flux density is 0.31 Jy beam 1; contours are 10%, 10%, 20%, 40%, and 80% of peak; and beam size is 0:77 0:19 mas at P.A. 50=2.
3. NOTES ON INDIVIDUAL SOURCES then bends to the south. At the end of the jet an extended 3.1. PKS 0208 512 component (or blend of components) appears to lie perpen- dicular to the jet direction. The orientation of this compo- Ground-based VLBI images of this source at 5 GHz are nent relative to the jet direction is strongly indicated in the reported by Tingay et al. (1996a) and Shen et al. (1998). closure phase information from triangles involving Both images show a bright core and a marginally resolved HALCA, indicating that this structure is not an artifact of extension along a position angle of 233 (angular resolution the ðu; vÞ coverage. of approximately 2 mas in both cases). The VSOP image (Fig. 1) resolves this source into a clear core-jet structure, in agreement with the lower resolution ground-based images 3.2. PKS 0438 436 (including the geodetic images from x 2.2); close to the core VLBI images at 2.3 GHz by Preston et al. (1989; 14 mas the jet emerges at a position angle of approximately 270 , resolution) and Murphy et al. (1993; no angular resolution
TABLE 3 Gaussian Best-Fit Models for the Brightest, Most Compact Component in Each Image
h Source S R (deg) a b/a (deg) Tb;G Tb-co;G co (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
0:2 inf inf PKS 0208 512...... 0.6 0.2 0.0 0 0.4 0:3 <0.6 50 0.4 0:3 0.8 0:6 9.9 0:1 0:1 0:1 0:4 PKS 0438 436...... 1.0 0.2 0.0 0 0.3 0:0 0.5 0:0 42 1.1 0:7 4.3 2:3 19.0 0:2 0:2 2:0 3:8 PKS 0537 441...... 0.8 0:3 0.0 0 0.3 0.1 0.8 0:3 67 0.6 0:3 1.0 0:7 9.4 0:6 0:3 1:5 2:4 PKS 0637 752...... 1.3 0:2 0.0 0 0.6 0:1 0.3 0.1 73 0.6 0:4 1.0 0:7 8.2 0:3 inf inf PKS 1104 445...... 0.4 0.1 0.0 0 0.3 0:1 <1.0 27 0.6 0:4 1.4 1:0 12.8 0:0 PKS 1127 145...... 0.4 0.1 4.3 97 0.2 0:1 <0.6 0 >0.6 >1.3 10.5 0:4 inf inf PKS 1424 418...... 0.9 0.2 0.0 0 0.2 0:0 <1.0 10 1.2 0:8 3.1 2:1 12.1 0:2 0:4 inf inf PKS 1610 771...... 0.9 0:5 0.0 0 0.8 0:5 <0.8 33 0.2 0:1 0.6 0:5 13.0 inf inf PKS 1622 253...... 0.6 0.2 0.0 0 <1.0 <1.0 60.2 0:1 0.3 0:1 8.8 0:2 5:1 9:5 PKS 1622 297...... 1.5 0.3 0.0 0 0.4 0.1 0.8 0:7 65 0.6 0:4 1.1 0:5 8.7 0:2 0:9 0:9 PKS 1718 649...... 1.3 0:3 0.0 0 1.0 0.4 0.4 0.2 62 0.1 0:05 0.1 0:05 5.0 0:7 0:7 1.1 0.3 6.7 134 1.3 0.5 0.4 0.2 49 0.1 0:07 0.1 0:07 5.0 0:3 0:3 PKS 2152 699...... 0.3 0.1 0.3 130 0.9 0.2 0.2 0.1 42 0.1 0:05 0.1 0:05 5.1 0:4 15:2 29:5 PKS 2255 282...... 2.7 0.4 0.0 0 0.3 0.1 0.4 0:2 27 3.6 2:6 7.0 5:2 9.6 0:2 PKS 2355 534...... 0.4 0:1 0.0 0 <0.3 <0.5 49 >0.4 >0.8 9.9 Note.—Col. (1): source; col. (2): S, the integrated flux density of the model component in Jy; col. (3): R, the distance of the model component from the designated phase center in milliarcseconds; col. (4): h, the position angle of the model component centroid from the designated phase center in degrees east of north; col. (5): a, the major-axis extent (FWHM) of the model component in milliarcsec- onds; col. (6): b/a, the ratio of model component minor axis to major-axis extent; col. (7): , the position angle of the model component major axis in degrees east of north; col. (8): Tb;G, the observed brightness temperature of the Gaussian component, in units 12 12 of 10 K; col. (9): Tb-co;G, the source-frame (comoving) brightness temperature of the Gaussian component, in units of 10 K; and col. (10): co, the frequency of the observed emission in the source frame in GHz. PKS 0208-512 PKS 0438-436 PKS 0537-441 PKS 0637-752 1997 Nov 21
PKS 0637-752 PKS 1104-445 PKS 1127-145 PKS 1424-418 1999 Aug 19
PKS 1610-771 PKS 1622-253 PKS 1622-297 PKS 1718-649
PKS 2152-699 PKS 2255-282 PKS 2355-534
Fig. 9.—Flux density vs. (u; v) distance for each of the sources—self-calibrated data and final model Fig. 10.—Top: Observer-frame Gaussian component brightness temperature estimates for the 14 sources observed with VSOP. Bottom: Source-frame Gaussian component brightness temperature estimates (i.e., corrected for redshift). In both plots, RG denotes radio galaxy. VSOP RADIO SOURCE OBSERVATIONS 325
TABLE 4 and the southern component is fully resolved on the base- Log of Geodetic VLBI Observations lines to HALCA; it shows a compact core and an extension to the east approximately 1 mas in length, in the direction Date of the extension seen from previous ground-based Range observations. Source Mean Date (days) Antennas
PKS 0208 512..... 1993.11 1 O, Hb, S, E, Dt 3.3. PKS 0537 441 PKS 0208 512..... 1993.73 20 O, Hb, S, E, F PKS 0537 441 has previously been imaged by Tingay et PKS 0208 512..... 1994.13 5 O, Hb, S, E, Dt PKS 0208 512..... 1994.73 6 Hb, S, E, Ft, F, K al. (1996a; 4.8 and 8.4 GHz, 7 and 2 mas resolution, respec- PKS 0208 512..... 1995.16 5 O, Hb, S, E, Dt, F tively) and Shen et al. (1998; 4.8 GHz, 3 mas resolution). All PKS 0537 441..... 1993.08 10 O, Hb, S, E, Dt three images show an approximately 4 mas northern exten- PKS 0537 441..... 1995.24 16 O, Hb, S, Dt, F, K, Ka sion to a bright, compact core. The value of the VSOP reso- PKS 0537 441..... 1995.53 1 Hb, S, E, Dt, F lution is again apparent in the new image (Fig. 2). The PKS 0537 441..... 1995.75 11 O, Hb, S, E, Dt, F, K, Dg extension to the core is now resolved into a jet that curves PKS 1610 771..... 1993.09 5 O, Hb, S, E, Dt over its 4 mas length. Close to the core the jet has a position PKS 1610 771..... 1993.44 1 Hb, S, E, Dt angle of approximately 60 , the jet then curves to the north, PKS 1610 771..... 1993.91 5 Hb, E, Dt, F in agreement with the lower resolution ground-based PKS 1610 771..... 1994.12 1 O, Hb, S, E, Dt images. This structure is also seen in 8.4 GHz global geo- PKS 1610 771..... 1995.12 10 O, Hb, S, E, Dt, F PKS 1610 771..... 1995.58 7 Hb, S, E, Dt, F detic VLBI images of PKS 0537 441, which are almost PKS 1610 771..... 1995.84 7 O, Hb, S, E, Dt, F resolution-matched to the VSOP image (x 2.2). Note.—Antenna designations: O ¼ O0Higgins; Hb ¼ Hobart; 3.4. PKS 0637 752 S ¼ Santiago; E ¼ Hartebeesthoek; Dt ¼ DSN, Tidbinbilla; F ¼ Fortleza; K ¼ Kokee; Ka ¼ Kashima; and Dg ¼ DSN, Goldstone. PKS 0637 752 was first imaged with VLBI by Tingay et al. (1998c) at 5 GHz with 2 mas angular resolution. The ground-based image shows a bright core component and a quoted) show that PKS 0438 436 consists of two compo- jetlike extension to the west of approximately 5 mas length, nents separated by approximately 35 mas along a position which consists of several components. Figures 2 and 3 show angle of 43 . Tingay et al. (1998b) presents a higher resolu- our two images of this source, at 4.976 and 4.816 GHz, tion (3 mas) VLBI image at 4.8 GHz that resolves the struc- showing in greater detail the components within 3 mas of ture of these two components and shows that both are the core. Table 3 lists the core parameters from the first elongated, but neither along the position angle that joins epoch of data (Fig. 2); the core parameters from the second them. A slightly higher resolution (2 mas) ground-based epoch gave a brightness temperature estimate very consis- image at 4.8 GHz by Shen et al. (1998) shows that the south- tent with the first-epoch data. These two observations were ern component is heavily resolved, whereas the northern approximately 20 months apart, and changes in the struc- component (presumably the core) remains compact. The ture of the jet are apparent in that time, although one needs VSOP image (Fig. 1) detects only the northern component, to be wary of the effects due to the different ðu; vÞ coverages. These images have already been discussed elsewhere in con- junction with geodetic VLBI images, ATCA observations, Hubble Space Telescope observations, and the first astro- nomical observations of the Chandra X-Ray Telescope (Schwartz et al. 2000; Chartas et al. 2000; Lovell et al. 2000b). Briefly, the VSOP and geodetic VLBI data yield proper motions of 0:41 0:03, 0:29 0:05, and 0:36 0:09 mas yr 1 for three components in the jet. The proper motions are just consistent with a weighted average appa- 1 rent speed of (17:8 1:0)c (H0 ¼ 70 km s and q0 ¼ 0:15; Schwartz et al. 2000; Lovell et al. 2000b).
3.5. PKS 1104 445 PKS 1104 445 was included in the first southern hemi- sphere VLBI Experiment observations (Preston et al. 1989); from limited data (13 mas angular resolution) they model the source as a 2.5 Jy, 4 mas FWHM Gaussian component with a 0.2 Jy pointlike secondary component approximately 17 mas away at a position angle of approximately 75 . Shen et al. (1997) imaged PKS 1104 445 at 5 GHz with a global array (3 mas angular resolution) and found a 1.79 Jy core and evidence for jet components within 3 mas of the core between position angles of 41 and 62 . Our VSOP data are consistent with previous observations on the Earth baselines but show the source to be heavily resolved on baselines Fig. 11.—Sample ðu; vÞ coverage for a geodetic VLBI observation of greater than 300 M at 5 GHz, leaving a compact compo- PKS 0208 512 at 2.3 GHz, from 1994.13. nent of 0.3 Jy on the longest space baselines (Fig. 3). 1993.11 1993.73 1994.13 1994.73 1995.16 326
Fig. 12.—Geodetic images of PKS 0208 512. Top: Sequence of five images at 2.3 GHz. Bottom: Sequence of five images at 8.4 GHz. The lowest contours for all images are 4%, and the highest contours are 64% of peak brightness. Peak brightness can be inferred from the model-fit flux densities of Table 5. 1993.08 1995.24 1995.53 1995.75 327
Fig. 13.—Geodetic images of PKS 0537 441. Top: Sequence of four images at 2.3 GHz. Bottom: Sequence of four images at 8.4 GHz. The lowest contours for all images are 4%, and the highest contours are 64% of peak brightness. Peak brightness can be inferred from the model-fit flux densities of Table 6. 1993.09 1993.44 1993.91 1994.12
1995.12 1995.58 1995.84
1993.09 1993.44 1993.91 1994.12
1995.12 1995.58 1995.84
Fig. 14.—Geodetic images of PKS 1610 771. Top: Sequence of seven images at 2.3 GHz. The lowest contours for the 2.3 GHz images, in temporal order, are 1%, 2%, 1%, 8%, 1%, 1%, and 1%, and the highest contours are 64% of peak brightness. Peak brightness can be inferred from the model-fit flux densities of Table 7. Bottom: Sequence of seven images at 8.4 GHz. The lowest contours for the 8.4 GHz images, in temporal order, are 4%, 8%, 2%, 2%, 2%, 2%, and 4%, and the highest contours are 64% of peak brightness. Peak brightness can be inferred from the model-fit flux densities of Table 7. VSOP RADIO SOURCE OBSERVATIONS 329
TABLE 5 TABLE 7 Geodetic VLBI Models for PKS 0208 512 Geodetic VLBI Models for PKS 1610 771
h h Date SR(deg) a Date SR(deg) a b/a (deg)
2.3 GHz 2.3 GHz
1993.11 ...... 1.6 0.0 0 0.5 1993.09 ...... 0.7 0.14 103 0 1 0 1.1 1.3 119 0.4 1.0 0.31 74 2.7 0.6 23 1993.73 ...... 1.9 0.0 0 0.8 1993.44 ...... 1.2 0.04 116 0 1 0 0.7 1.3 128 0.9 2.1 0.41 38 3.0 0.7 6 1994.13 ...... 1.9 0.0 0 0.2 1993.91 ...... 1.2 0.01 129 0.7 0.7 44 1.6 1.4 133 1.3 0.1 1.3 37 0 1 0 1994.73 ...... 2.9 0.0 0 0.7 1995.12 ...... 1.1 0.06 175 0 1 0 1.9 1.1 134 1.4 1.6 0.53 27 3.2 0.6 37 1995.16 ...... 2.8 0.0 0 0.9 0.1 4.1 22 0 1 0 0.4 1.3 122 0.0 1995.58 ...... 1.5 0.01 163 0 1 0 1.6 0.76 33 5.0 0.4 36 8.4 GHz 1995.84 ...... 1.5 0.02 23 0 1 0 1993.11 ...... 2.4 0.0 0 0.5 2.3 0.24 23 4.0 0.5 46 1993.73 ...... 2.5 0.0 0 0.5 8.4 GHz 1994.13 ...... 3.0 0.0 0 0.5 0.6 0.7 114 0.1 1993.09 ...... 0.5 0.0 0 0 1 0 1994.73 ...... 3.2 0.0 0 0.4 1993.44 ...... 1.7 0.0 115 0.4 1 0 1995.16 ...... 2.4 0.0 0 0.4 0.2 0.8 135 0 1 0 0.4 0.7 108 0.2 1993.91 ...... 0.9 0.0 74 0.2 1 0 0.1 0.8 135 0 1 0 Note.—Cols. (2)–(5) are as for Table 3. 1994.12 ...... 2.5 0.0 54 0.4 1 0 0.3 0.9 135 0 1 0 1995.12 ...... 1.7 0.0 132 0.3 1 0 3.6. PKS 1127 145 0.4 1.1 138 0 1 0 PKS 1127 145 is a well-studied quasar that has been 1995.58 ...... 0.8 0.0 130 0 1 0 investigated with VLBI by several authors at frequencies 0.2 1.2 132 0 1 0 between 1.7 and 22 GHz (Jorstad et al. 2001; Tingay et al. 1995.84 ...... 1.9 0.0 47 0.4 1 0 0.2 1.2 132 0 1 0 1998a; Shen et al. 1998; Kellermann et al. 1998; Fey, Clegg, & Fomalont 1996; Bondi et al. 1996; Wehrle et al. 1992; Lin- Note.— Cols. (2)–(7) are as for Table 3. field et al. 1989, 1990). The source basically consists of a strong east-west double plus a weak extension to the north- east (Wehrle et al. 1992) and complicated emission between element, at both 2.3 and 15 GHz. Vermeulen & Cohen the double components (Jorstad et al. 2001). Linfield et al. (1994) listed a proper motion of 0:00 0:02 mas yr 1 in (1989, 1990) detected PKS 1127 145 on 14,000 km space their compilation of proper motion statistics. The 22 GHz VLBI baselines using the TDRSS satellite as the orbiting monitoring of Jorstad et al. (2001) clarifies the situation somewhat; they measure stationary and moving compo- nents ranging up to (19:8 6:1) h 1c. Our VSOP image (Fig. 4) shows the structure that is con- TABLE 6 sistent with previously published ground-based images; a Geodetic VLBI Models for PKS 0537 441 strong double with a northeast extension and emission h between the double components. Figure 4 is annotated with Date SR(deg) a the component designations from Jorstad et al. (2001), which are discussed below. 2.3 GHz The brightness temperature of the presumed core (at the 1993.08 ...... 3.5 0.0 0 2.1 western end of the source), component A (Jorstad et al. 1995.24 ...... 3.5 0.0 0 1.0 2001), is listed in Table 3. 1995.53 ...... 4.2 0.0 0 1.4 To investigate the motions of components in PKS 1995.75 ...... 3.6 0.0 0 1.4 1127 145, we have compiled all VLBI observations of PKS 1127 145 from the literature, at different frequencies 8.4 GHz (Jorstad et al. 2001; Tingay et al. 1998b; Shen et al. 1998; 1993.08 ...... 2.6 0.0 0 0.3 Kellermann et al. 1998; Fey et al. 1996; Bondi et al. 1996; 0.5 0.4 95 0.1 Wehrle et al. 1992), to produce an overall temporal history 1995.24 ...... 4.2 0.0 0 0.1 of the milliarcsecond-scale component positions in this 2.3 0.5 88 0.7 source, shown in Figure 16. Included in this compilation are 1995.53 ...... 5.5 0.0 0 0.2 the VSOP data from this work. We have adopted errors 1.5 0.5 117 0.0 from the literature where given, or half the beam size where 1995.75 ...... 3.7 0.0 0 0.2 errors have not been given. For the VSOP data, we mea- 1.1 0.4 90 0.2 sured the positions of components in the image plane in Note.—Cols. (2)–(5) are as for Table 3. DIFMAP and adopted the full beam size as the error, in rec- Fig. 15.—Core-component separations as a function of time and linear least-squares fits for the apparent proper motions in PKS 0208 512 (top), PKS 0537 441 (middle), and PKS 1610 771 (bottom), from the geodetic and VSOP data sets. VSOP RADIO SOURCE OBSERVATIONS 331
Fig. 16.—Positions of milliarcsecond-scale components in PKS 1127 145 relative to the core component, as a function of time, compiled from the literature and this work. Component designations are from Jorstad et al. (2001). Solid lines are weighted linear least-squares fits to the data. ognition of the sparse ðu; vÞ coverage of the VSOP data. In cantly different from that found by Jorstad et al. (2001). The Figure 16 the VSOP data are the points at 1999.31. high signal-to-noise points from the 22 GHz monitoring of Observations of PKS 1127 145 prior to 1994 were Jorstad et al. (2001; 1996.60, 1996.90, 1997.58) dominate model-fitted with a two-component model at 1.6 GHz the weighted linear least-squares fit to the C2 component (Bondi et al. 1996; 1980.10, 1981.80, and 1987.90) and 5.0 positions, shown in Figure 16. The 15 GHz image of Keller- GHz (Shen et al. 1998; 1993.36). In Figure 16 these two mann et al. (1998; 1997.28) and the 8.4 GHz image of components are taken to be dominated by components A Tingay et al.(1998b; 1997.13) are consistent with the Jorstad (the core) and C2. The 5.0 GHz image of Wehrle et al. et al. (2001) data for C2. The VSOP data and historical data (1992; 1988.59) was not model-fitted, but the two compo- from the literature indicate that components C1 and C2 nents are clearly resolved, allowing an estimate of the com- have apparent proper motions of 0:24 0:10 and 1 ponent separation from the published image. Likewise, the 0:14 0:06 mas yr , corresponding to app ¼ð9:1 3:8Þ 1 1 8.6 GHz image of Fey et al. (1996; 1994.52) is well resolved, h and app ¼ð5:3 2:3Þ h , respectively (over the course showing the two components; however, Fey et al. (1996) of almost 20 yr for C2). Previously Jorstad et al. (2001) sug- provide model-fit positions for these components, which gested that C1 and C2 are stationary components, based on have been used in Figure 16. The identification for the com- three 22 GHz observations over a period of 1 yr. ponent 1 mas from the core at 1994.52 (Fey et al. 1996) is The estimate of 0:27 0:05 mas yr 1 for component B2, 1 unclear. corresponding to app ¼ð10:3 1:9Þ h , from Jorstad et The VSOP image in Figure 4 has detected components A, al. (2001) remains unchanged. Finally, from two epochs of B1, B3, C1, and C2 at 1999.31. The positions of these com- data, the proper motion for component B3 is 0:08 0:13 1 1 ponents derived from the VSOP data generally fit well into mas yr , corresponding to app ¼ð3:0 4:9Þ h . the overall picture of an expanding jet source. The fact that Further high-frequency and high-resolution VLBI moni- component B2 was not detected is interesting, but it can be toring is required to confirm the motions of all milliarcsec- noted that Jorstad et al. (2001) previously observed another ond-scale components in PKS 1127 145. component, D1, emerge from the core and fade to below the noise level at approximately the same position that B2 PKS 1424 418 appears to have faded. Even though Figure 16 contains 3.7. position estimates at a number of different frequencies and PKS 1424 418 was modeled by Preston et al. (1989) as a resolutions, which should prompt caution in such a compar- 1.4 Jy core and a 0.26 Jy secondary component a distance of ison, there appears to be good evidence that all milliarcsec- 23 mas from the core at a position angle of 56 , from 2.3 ond-scale components in PKS 1127 145 are moving away GHz VLBI with an angular resolution of 13 mas. Shen et al. from the core. (1998) imaged the source at 5 GHz with 2 mas resolution The weighted linear least-squares fit to the compiled data and found a 1.36 Jy core with a 0.12 Jy secondary, 2.74 mas for component B1 gives a proper motion of 0:42 0:08 mas distant at a position angle of 260 . Our VSOP image (Fig. 4) 1 1 yr , corresponding to app ¼ð16:0 3:0Þ h , not signifi- shows a compact component with a jetlike extension at a 332 TINGAY ET AL. Vol. 141 position angle close to the position angle noted by Preston angle of 22 , almost exactly 90 offset from the VLBI jet et al. (1989). position angle (S. J Tingay & B. Punsly 2002, in prepara- tion). This extended structure was first noted by Perley 3.8. PKS 1610 771 (1982). There are no detailed ground-based VLBI observations of PKS 1610 771 in the previous literature. Preston et al. 3.11. PKS 1718 649 (1989) modeled the source as a 3.8 Jy component 10 mas in extent with a 1.4 Jy halo approximately 50 mas in diameter, PKS 1718 649 is the lowest redshift GHz-peaked spec- using data with 22 mas resolution. Our VSOP data confirm trum radio source (Tingay et al. 1997). The radio structure the presence of a very extended component detected only on is approximately 7 mas in total extent and, as is typical for Earth baselines. However, the small-scale structure is GPS galaxies, consists of two almost equally bright compo- resolved into three components at a position angle of nents. The VSOP data we obtained for this source are sparse approximately 30 (Fig. 5). Table 3 lists the southern com- and could not be easily imaged, probably because the com- ponent, which is the brightest and most compact compo- bination of sparse (u; v) coverage and highly resolved struc- nent detected, but it is not necessarily the core. The ture (x 2) is not well suited to the clean algorithm. However, interpretation of the dual-frequency geodetic data of x 2.2 is a good model fit was possible (Fig. 6) and shows that the consistent with this component being the stationary core two components are resolved on space baselines. The component. ground-based images of Tingay et al. (1997) and the space Comparison of the geodetic data and the VSOP data indi- VLBI model agree extremely well. The measured separation cate a complicated variable, evolving, expanding structure of the two components has remained the same (approxi- with different spectral components. mately 7 mas), to within the errors (approximately 0.5 mas) over a period of 6 yr, implying an upper limit on the separa- 3.9. PKS 1622 253 tion speed of the components of 0.08c [cf. (0:05 0:2)c for PKS 1934 638, another low-redshift GPS galaxy (Tzioumis PKS 1622 253 has previously been imaged with VLBI by et al. 1989)]. Since both components in PKS 1718 649 are Fey et al. (1996) at 2.3 and 8.4 GHz (9 and 3 mas resolution, similar in brightness temperature and neither is likely to be respectively), Tingay et al. (1998a) at 4.8 GHz (2 mas resolu- the nucleus, both components are listed in Table 3. Typi- tion), and Jorstad et al. (2001) at 22 GHz (0.8 mas resolu- cally the core components in GPS sources are very weak. A tion). From the ground-based images of Tingay et al. higher sensitivity (and possibly higher frequency) VLBI (1998c) and Jorstad et al. (2001) a strong core and weak jet- observation would be required to detect the core component like extension is evident, along position angle 300 . in PKS 1718 649. Jorstad et al. (2001) have measured the apparent speed of 1 this jetlike extension to be app ¼ð14:1 3:6Þ h . Our VSOP image (Fig. 5) shows that the core component is mar- 3.12. PKS 2152 699 ginally resolved. The extended jet is completely resolved in PKS 2152 699 is a low-redshift Fanaroff-Riley type II the VSOP image but detected on the Earth baselines. PKS radio galaxy with strong nuclear radio emission. The 1622 253 has been shown to display intraday variability nuclear radio source has been investigated with VLBI, and (Bignall et al. 2002). its relationship to the larger scale radio and optical structure of the galaxy has been discussed (Tingay et al. 1996b). The PKS 1622 297 3.10. VSOP image (Fig. 7) shows a resolved core and highly lin- PKS 1622 297 is the most luminous (assuming isotropic ear, narrow jet approximately 6 mas in extent. The position emission) gamma-ray source ever detected (Mattox et al. angle of the jet is in excellent agreement with that found by 1997) and has previously been imaged with VLBI at 1.7 and Tingay et al. (1996b). 5.0 GHz by Tingay et al. (1998a) and at 15, 22, and 43 GHz by Jorstad et al. (2001). The ground-based images at all fre- PKS 2255 282 quencies are consistent and show a series of components 3.13. along a position angle of 69 . Jorstad et al. (2001) find a PKS 2255 282 came to attention as a flaring gamma-ray weak stationary feature 15.4 mas from the nucleus at 15 source (Bertsch et al. 1998). Tornikoski et al. (1999) pub- GHz (which is stronger in the 5.0 GHz image of Tingay et lished multiwavelength data, including ground-based VLBI, al. 1998b). Jorstad et al. (2001) also find components at 3 for this quasar. The VLBI image of Tornikoski et al. (1999; and 5 mas from the core that have apparent speeds of 10 3 mas resolution) shows a strong core component and a jet h 1c and components within 1.5 mas with apparent speeds extending approximately 5 mas to the southwest. Our VSOP between 5 and 10 h 1 c. The VSOP image (Fig. 6) of PKS image (Fig. 7) shows the strong core component; the 1622 297 shows a strong core and a weaker component extended jet is heavily resolved, although weakly apparent approximately 1.5 mas from the core, consistent with the in the image. position of component D1 identified from the 22 GHz mon- itoring of Jorstad et al. (2001); much weaker jet structure is 3.14. PKS 2355 534 apparent farther from the core, close to the jet position angle of 69 . PKS 1622 297 has also been shown to dis- The 5 GHz VLBI observations of Shen et al. (1998; 3 mas play intraday variability (Bignall et al. 2002). An ATCA resolution) show PKS 2355 534 to have two components, image of PKS 1622 297 at 4.8 GHz reveals a bright core of 1.54 and 0.20 Jy, of equal size and separated by 4.88 mas. with strong extended emission in the form of two compo- Our VSOP data proved insufficient to produce an image. nents, placed equal angular distances either side of the core Figure 8 shows a single-component model fit of the data, (700) with similar sizes and brightnesses, along a position which appears in Table 3. No. 2, 2002 VSOP RADIO SOURCE OBSERVATIONS 333
4. CONCLUSIONS al. (2001b), will appear elsewhere. However, the lack of We have presented the highest resolution images and obvious correlation agrees with the results of Lister et al. model fits yet for the majority of the 14 southern hemisphere (2001b), who, in a full statistical analysis of a better defined, compact radio sources investigated here. Model-fitting of larger, and more complete sample of sources, found that the most compact components of these sources (in most VSOP-derived brightness temperatures did not correlate cases corresponding to the core) show that the two low- with any standard indicators of relativistic beaming, only redshift radio galaxies PKS 1718 649 (GHz-peaked spec- intraday variability (IDV). The conclusion of Lister et al. trum) and PKS 2152 699 (FR II) have the lowest peak (2001b) was that IDV was also correlated with relativistic brightness temperatures, as expected since neither are likely beaming indicators such as radio spectral index, optical to be highly Doppler boosted. However, the two galaxies emission line equivalent width, and source size. The inter- are less luminous and closer than the quasars in the sample pretation of this conclusion is complicated by the fact that and in PKS 1718 649 we have not likely detected the core most IDV is thought to be caused by a mechanism extrinsic component. Therefore, factors other than differences in to the source, scintillation due to a screen of material Doppler boosting may cause the brightness temperatures of between the source and observer (e.g., Bignall et al. 2002). the galaxies to be systematically lower than for the quasars. Although some of the southern objects described in this Of the sources optically identified as quasars, six have paper are known IDV sources, overall there are not enough been detected in greater than 100 MeV gamma rays by the IDV monitoring observations of the full sample to perform EGRET instrument aboard the CGRO and six have not a test similar to that done by Lister et al. (2001b). been detected by EGRET. One of the chief goals of these In terms of variability, three of the gamma-ray–loud qua- VSOP observations was to compare the properties of sars (PKS 0537 441, PKS 1622 253, and PKS 2255 282) EGRET-identified and gamma-ray–quiet AGNs (see dis- are far more variable (Table 1 and Fig. 17) than the other cussion in Tingay et al. [1996a, 1998a, 1998b, 1998c]); to see sources. PKS 0537 441 and PKS 1622 253 are known as why such a comparison is interesting). The sample of probable IDV sources and the variability may be due in part gamma-ray–loud sources can be considered close to a flux to interstellar scintillation rather than intrinsic mechanisms. density limited (at gamma-ray energies) sample, while the While not significant in such a small sample, analysis of the gamma-ray–quiet sources have been selected to be represen- full ATCA monitoring database should be undertaken to tative of the parent population of gamma-ray–loud sources, determine whether there is a significant difference in the var- based on radio and optical properties. From Table 3 and iability properties of gamma-ray–loud and gamma-ray– Figure 9, a comparison of the brightness temperature distri- quiet AGNs. PKS 2255 282 also has by far the most butions for these two classes of source shows no obvious dif- inverted radio spectrum of all the sources. ferences; a range in observer’s frame brightness temperature The VSOP work reported here has been particularly use- between approximately 1:0 1012 and 1:2 1012 K is con- ful in analyses of the structural evolution of five sources, sistent with the measurements for all of the quasars and like- PKS 0208 512, PKS 0537 441, PKS 0637 752 (Schwartz wise, in the source frame, brightness temperatures between et al. 2000; Chartas et al. 2000; Lovell et al. 2000b), PKS approximately 2:0 1012 and 3:3 1012 K are consistent 1127 145, and PKS 1718 649. Geodetic VLBI data have with all sources. These results are consistent with those of also proved extremely useful in these investigations. Tingay et al. (2001) and Lister et al. (2001b), who found no differences in source frame brightness temperatures between We gratefully acknowledge the VSOP Project, which is EGRET-identified and gamma-ray–quiet AGNs from a led by the Japanese Institute of Space and Astronautical Sci- study of the Pearson-Readhead VLBI sample using the ence in cooperation with many organizations and radio tele- VSOP facilities. scopes around the world. The Australia Telescope is funded Also, considering the quasars, we find no evidence for by the Australian Commonwealth Government for opera- relationships between VSOP-derived brightness tempera- tion as a national facility managed by CSIRO. Part of this tures and average values of the radio flux density, spectral work was undertaken at the Jet Propulsion Laboratory, index, percentage polarization, and variability index as California Institute of Technology, under contract with the tabulated in Table 2 (and shown graphically in Figure 17). National Aeronautics and Space Administration while This was also the case when considering the measurements S. J. T. held an NRC/NASA-JPL Research Associateship. of these parameters taken closest in time to each VSOP The Astronomical Image Processing Software (AIPS) and observation, rather than considering average values. The the VLBA were developed and are maintained by the small number of sources to consider and the abundance of National Radio Astronomy Observatory, which is operated only lower limits in the data may make the detection of sig- by Associated Universities, Inc., under cooperative agree- nificant correlations between VSOP-derived brightness tem- ment with the National Science Foundation. We thank the perature and other parameters difficult; a full statistical appointed referee, Ed Fomalont, for a careful review of the analysis of these data, similar to that performed by Lister et manuscript and suggestions that improved the paper. Fig. 17.—Plots of quantities derived from ATCA monitoring observations for the quasars (modulation index, percentage linear polarization, flux density, and spectral index) against VSOP-derived brightness temperatures. ATCA data are described in the text and tabulated in Table 1. The EGRET-identified quasars are represented by the dashed lines and the gamma-ray–quiet quasars by the solid lines. VSOP RADIO SOURCE OBSERVATIONS 335
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